Current driving circuit



r R F F. E. GOODWIN CURRENT DRIVING CIRCUIT 2 Sheets-Sheet l llll R 1L gMarch 20, 1962 Filed Dec. 1, 1959 Q? 2E 3 -62: 23m

FRANCIS E GOODWIN,

INVENTOR BY ATTORNEY March 20, 1962- F. E. GOODWIN 3,026,454

CURRENT DRIVING CIRCUIT Filed Dec. 1, 1959 2 Sheets-Sheet 2 VOLTAGEVOLTAGE CURRENT w FRANCIS E. cooowm, INVENTOR s6 *54 *sz 50 CURRENTAMPERES ATTORNEY United States "neat 3,026,454 Patented Mar. 20, 1962This invention relates to current driving circuits and particularly to atransistor current source for ferrite phase shifters and other inductiveloads.

An inductive element when being driven with a step source of impedance Rhas a basic time constant L/R for changing current through the inductiveelement where L is the inductance of the element. For the time constantof changing current through the inductive element to be small, eitherthe inductance L must be small or the effective resistance R must belarge, or both. The value of the inductance L may be fixed by otherconsiderations such as when utilizing a ferrite phase shifter where theL may be established at a fixed value.

One prior art means to drive ferrite phase shifters with a small timeconstant is to use high resistance vacuum tubes as current drivers.However, this type of driving arrangement requires a very large currentand wastes a relatively large amount of power, especially if a largeconstant current is to be maintained through the inductance of the phaseshifter for a long period of time. In circuit arrangements where a largenumber of phase shifters are utilized, the large power loss makes vacuumtube drivers impractical.

Another conventional driver for inductive loads such as phase shiftersis a current source utilizing power transistors which will effectivelyhold a constant current indefinitely. However, the conventionaltransistor arrangement provides a source impedance of only a few ohmsand the power source has a relatively small potential so that a verylarge time constant is present when changing the current through a phaseshifter, for example. A conventional regulated current source utilizingtransistors may include a switching transistor coupled in series withthe inductance and a transistor utilized as a regulating amplifiercoupled between a first and a second resistor that are connected betweenthe current source and the inductance and the base of'the switchingtransistor. The voltage rating of the power source and the relative sizeof the two resistors determines the current passed through theinductance.

A current source utilizing transistors that would provide high voltageswithout destroying the transistors and high impedances to reduce thetime constant so as to rapidly change from one current level through theinductance to another would be very valuable to the art.

It is thus an object of this invention to provide a current source forinductive loads utilizing transistors elements that is highly efiicientbecause of requiring a minimum amount of power.

It is a further object of this invention to provide an efficient currentsource for switching the current from one level to another through aninductive load with a minimum of time delay.

It is a still further object of this invention to provide a source ofdriving current for shifting the phase of a phase shifter element byrapidly changing the current to a plurality of predetermined levels.

It is another object of this invention to provide an efficient currentsource for a phase shifter that utilizes transistors and that utilizesswitching transients for changing current through the inductive load ofthe phase shifter with a minimum of delay.

Briefly, this invention is a current source for inductive loads thatincludes a driving transistor having its load current path coupled inseries with the inductive load. A regulating transistor has its loadcurrent path coupled across a power source through a protective diodefor applying a potential to the base of the driving transistor duringsteady state conditions. The base of the regulating transistor iscoupled through a programming signal generator to a point between afirst and a second resistorconnected between the power supply and theinductive load. The programming signal generator responds to a source ofsignals to develop switching signals having predetermined volta-gelevels corresponding to currents required through the inductive load.These voltage levels control the regulating transistor to maintain thedesired current through the inductive load during steady stateconditions and respond to a negative feedback signal from the inductiveload to maintain the regqilating transistor biased so that a constantcurrent is maintained through the inductive load for each steady stateor direct current voltage level of the switching signal. To providelarge voltage transients when the switching signals change voltagelevel, a transient amplifier is coupled in series between the signalgenerator and the base of the driving transistor for applying switchingtransients signals to the base of the driving transistor and presentinga high impedance to the inductive load during switch ing transients of afirst polarity. A diode is coupled in series in the load current pathsof the driving transistor for providing a high impedance to theinductive load during switching transients of a second polarity. Theswitching transients of both polarities are applied to the inductiveload to provide rapid changes of current therethrough. Thus, the currentdriving circuit of this invention maintains a regulated steady statecurrent through the inductive load and during transient conditions,rapidly changes the current through the load to a new value.

The novel features of this invention, as well as the invention itself,both as to its organization and method of operation, will best beunderstood from the following description, taken in conjunction with theaccompanying drawings, in which like characters refer to like parts,

' and in which:

' FIG. 1;

FIG. 3 is a graph of voltage versus time showing a representativesequence of voltages developed by the programming signal generator ofFIG. 1.

FIG. 4 is a graph of voltage versus time showing the transient andsteady state volt-ages developed by the transient amplifier of FIG. 1 inresponse to the voltages of FIG. 3;

FIG. 5 is a graph of current versus time for illustrating the rapidchange of current through the phase shifter of FIG. 1.

Referring first to FIG. 1 which shows a schematic diagram of the currentdriving circuit of this invention, the arrangements of the elementstherein will be described. A battery 10 having one end coupled to aground lead 12 as a voltage reference and the other end coupled to alead 14 is provided as a power source and may, for example, develop +6volts. The lead 14 applies power through a series connected firstresistor16 and second resistor 18 and through a lead 19 to one end of aninductive load or coil 20 of a phase shifter 22 that may be utilizedwith the circuit of this invention. The phase shifter 22 may include thecoil 20 wound about a rectangular waveguide (not shown) with an axiallydisposed ferrite rod and an axial magnetic field provided by thesolenoid. Suitable means are provided to prevent eddycurrent losses inthe Waveguide itself. A phase shifter I of this general type isdescribed on page 1510 of the November 195.7 issue of the IREproceedings in an article by F.. Reggia and E. G. Spencer entitled A NewTechniq-ue. in Ferrite Phase Shifting for Beam Scanning of MicrowaveAntennas.

The other end of the inductive load 2% passes current through theemitter to collector current path of a driver transistor 24 which is ofthe p-n-p type. The collector of the transistor 24 isv coupled throughthe anode to cathode path of a blocking diode 26 to the ground lead 12.

For steady state regulation of current through the inductive load 20, aregulating transistor 28 of the p-n-p type is provided. The emitter ofthe transistor 23 receives current from the lead 14 through the anode tocathode path of a zener diode 3,0 for providing a constant voltage dropso that the regulating transistor 28 operates in a desired amplificationrange. The collector of the transistor 28 is arranged to apply currentto the ground lead 12 through the anode to cathode path of a protectivediode 34 and through a lead 36 and a signal developing resistor 38, Thepotential developed by the resistor 38 is applied through a lead 40 tothe base of the driving transistor 24 for controlling the currentthrough the inductive load 2.0..

The base of the transistor 28.. is arranged to respond 'to the voltagedeveloped by a programming signal genorator. 4.4 which includes astepping switch 46 having an arm 48 movable to form a closed circuitwith contacts 59, 52, 54 and 56. A precision resistor 58 is providedwith tapped points, connected to the contacts 50, 52, 54 and 56 toprovide different potential levels. One end of the resistor 5,8 isvcoupled to the positive terminal of a battery 62. through a lead 64. andthe other end of theresistor 5,8 is biased from the other end of thebattery 62. The battery 62 may be selected to develop +2 volts. Thelead. 64 is biased from a point between the resistors 16-, and 18 sothat a change of current through the load effectively provides anegative feedback signal from the inductive load 20.

The arm 48 of the stepping. switch 46, is responsive through amechanical connection 66 to thestate of rotation of a scan motor 63 forexample. Thus, the arm 48 may sequentially move, for example, from thecontacts 50, 52, 54, 56v and back to the contact 50. The scan motor 68,may include an intermittent gear arrangement such as a rocket drive thatallows. the movement to occur rapidly from one contact to another aftera predetermined amount of rotation of the motor. It is to be noted thatthe continuous sequence; from a higher voltage to decreasing voltagelevels then back to the higher voltage level is only one sequence thatmay be provided by the signal generator 64.. By connecting the contactssuch as 50 to different tapping points of the resistor 58, any desiredsequence may be developed.

The signal applied to the arm 48. is. in turn applied through a lead/70to the base of the transistor 28 and to an amplifier 72 of a transientamplifier 74. The amplifier. 72 may be either an A.C. (alternatingcurrent) or a DC. (direct current) amplifier. The D.C. amplifier 72vappliesan amplified signal through a lead 76 to a high pass filter 78also included in the transient amplifier 74. The high pass filter 78applies only a transient signal to the lead 40 in response to. a changeof voltage level of the switching signal developed by the signalgenerator 44, as will be explained subsequently.

The high pass filter 78 may be any conventional type of filter and mayinclude a capacitor 80 having one end coupled to the lead 76 and theother end coupled to ground through a resistor 82 and to one end of asecond capacitor 84, The other end of the second capacitor 34 is coupledto thelead 40 and to groundthrough a resistor 86, thus passing onlytransient signals of short duration.

Referring now to FIG. 2 showing a graph of phase versus current for thephase shifter 22, the operation of the circuit will be explained infurther detail. A curve 88, which may be determined experimentally forany selected phase shifter 22 or any inductive load shows the requiredcurrent through the inductance coil 20 for four phase conditions of amicrowave signal applied through a waveguide (not shown) of the phaseshifter 22. Currents i 1' 1' and i indicate the current required to bepassed through, the phase shifter 22 to develop a phase shift of 0degree, 120 degrees, 240 degrees and 360' degrees.

Referring now to FIG. 1 and to FIG. 3, the latter of which shows thestep voltage developed by the signal generator 44,, to FIG. 4 whichshows the transient signals developed by the transient amplifier 74 andto FIG. 5 which shows waveforms of the currents passed through the phaseshifter 22, the operation of the circuit will be further explained. Thesequence of step or switching voltages of a waveform in FIG. 3 isdetermined by the amount of phase shift desired of the phase shifter 22.The voltages e e e and e of the waveform 90 are developed. at.respective times t through A, by the contact arm 48, respectively,moving in response to the scan motor 68 to contacts 50, 52, 54 and 56.

Before explaining the transient signals. developed when the switchingvoltages of FIG. 3 change voltage level, the steady state regulatingoperation of the, circuit will be explained. Each, switching voltagelevel of the waveform 90 such as the voltage 6 is applied to the base ofthe transistor 28 to bias the transistor 28 to pass a predeterminedcurrent from its emitterto collector and through the diode, 34 and thesignal forming resistor 38. In response to each, voltage level of the:waveform 90, a predetermined voltage is developed on the lead 40 toestablish a desired current through the emitter to the col lectoIT ofthe driver transistor 24. Thus,v in. response to each higher Voltagelevel e e e or 6 of the waveform 90,, the impedance through the'drivertransistor 24 is increased and the, current through theinductive load 20is decreased to the respective currents 1' i 1' or 1' as indicated bythe waveform 88 of FIG. 2.

In order to maintain a desired current level such as through theinductive load 20, to overcome variations of the Source. of potentialsuch as the battery 10, and variations of. the characteristics of theelements in the circuit such as. by temperature changes, a negativefeed: back arrangement is also. provided. The size of the resistors 16and 18 is designed so that for each voltage level of thewaveform 90 apredeterminedvoltage is developed on the lead 64, so that the desiredcurrent is supplied to the inductive load 20. In order to maintain thiscurrent, a change of current through the resistors 16 and 18 isreflected as a change of. voltage. through the resistor 58 and on thebase, of the transistor 28-. Thus, the potential" on the lead 40-ischanged to correct the impedance of the transistor 24 so as to maintainthe: desired current through the inductive load 20. For example, adecrease of current through the inductive loadv 20' causes an increaseof potential onthe lead 64 and a resultant increase of potential on thebase of the. transistor 28.

Thus, the current through the resistor 38. decreases with Referring nowprimarily to FIGS. 1, 3 and 5, the

transient operation of the circuit in accordance with this inventionwill be explained. When the voltage level of the switching signal of thewaveform 90 changes such as at times t to L, as a result of the contactarm 48 moving to a difierent contact 50 through 56, a transient signalis applied through the lead 70 to the amplifier 72 of the transientamplifier circuit 74. This transient signal is amplified in theamplifier 72 to form signals similar to transient signals 92, 94, 9'6,98 and 100' of a waveform 102 of FIG. 4. The amplifier transient signalssimilar to the signal 94 are applied to the high pass filter 78 andpassed therethrough as current signals to the lead 40. The high passfilter 78 only passes current signals therethrough during the time ofoccurrence of the transient signals such as 94, acting to block allsignalsduring steady state conditions. The current associated with thetransient signals such as 94 passes through the lead 4%) and theresistor 38 to develop the transient voltage signals 92, M, 96, 98 or100 on the lead v40. It is to be again noted that the transient signalssuch as 94 have a relatively short time of duration.

The elfect of a positive transient signal such as 94, 96 or 98 resultingfrom an increase in voltage level of the switching voltage of thewaveform 947 of FIG. will now be explained. The positive transientsignal such as 94 applied to the lead 40 rapidly biases the transistor24 into a nonconductive state to uncouple the inductive load 29 from theground lead 12. Further, the positive potential of the switchingtransient such as 94 is applied tothe emitter of the transistor 24 andto the end of the inductive load 20 coupled thereto. This positivetransient signal 94 rapidly decreases the current through the inductiveload 20 by effectively presenting a high impedance thereto so that thecurrent level decreases in a short period of time to a current level ofi Because of the short time interval of the transient signal such as 94,the current only decreases to the level i rather than to a lower currentlevel and then the transient signal is terminated allowing the normalregulating action of the circuit to maintain the current level i Thus,only transient signals of short duration are effectively passed throughthe high pass filter 78.

Without the transient amplifier arrangement in accordance with thisinvention, the characteristic delay in changing current through aninductive element would cause the current to change to the new level asindicated by a dotted waveform 6. Thus, the current driver of thisinvention allows the phase developed by the phase shifter 22 to berapidly changed without an extensive time delay. It is to be noted thatwithout the transient amplifier circuit 74 or" this invention, a currentdriver is limited to a voltage for effectively causing the change ofcurrent no greater than the power source or battery 10. The drivertransistor 24 is not damaged by exceeding its power limitations becauseit is biased out of conduction during the presence of the positivetransient signal. It is to be noted that the driver transistor 24 mustbe selected so that the voltage of the transient signal does not exceedits characteristic voltage limit. At the same time, the positivetransient signal such as 94 is impressed on the cathode of the diode 34biasing it out of conduction. Thus, the regulating transistor 28 isprotected from current passing in a direction opposite to its emitter tocollector path and exceeding the power rating of the transistor 28.

The operation of the circuit in response to a negative switchingtransient signal such as 100 will now be explained. A negative transientsignal such as 100 biases the driver transistor 24 in a conductive stateso as to act as a closed circuit. However, the negative transient signalis impressed on the anode of the diode 26 biasing it out of conductionand de-coupling the inductive load 20 from the ground lead 12. At thesame time, the negative transient signal such as 100 is impressedthrough the transistor 24 to the end of the inductive load 20 connectedto the emitter of the driver transistor 24. Thus, current is rapidlyincreased through the inductive load 20 from the current level i to thenew current level i The high impedance of the diode 26 and the negativetransient signal applied to the inductive load 20 develops a largeeffective impedance so that the time constant is very small for therequired current change. Similar to the positive transient signal, thenegative transient signal has a short time duration so that the currentis only changed to the desired. higher current level i The negativeswitching transient appears equally on the emitter, base and collectorof the driver transistor 24 so that the voltage rating is not exceeded.Also, because the diode 26 is biased out of conduction current isprevented from passing through the driver transistor 24 to exceed itspower rating. Although the negative transient signal such as 100 causesa relatively large current flow through the transistor 2% for a shortperiod of time, the current is limited by the resistor 38 so that thepower rating of the transistor 28 is not exceeded. Thus, the transistorelements in the circuit are protected from overload and damage.

It is to be noted that regardless of the sequence of voltages developedby the signal generator 44, the transients act to rapidly change thecurrent through the inductive load 2 to different current levels and theregulating portion of the circuit maintains the steady state currenttherethroug-h further corrected by the negative feedback arrangement.Because of the short time duration and large amplitude of the transientsignals, large current changes may he made in large inductive loads at avery rapid rate. It has been found that the circuit in accordance withthis invention has reduced the rise time of phase shifter current from300 microseconds to 10 microseconds utilizing a maximum transientvoltage of 60 volts. If the transistors are able to withstand highervoltage and power ratings or if means are utilized to protect thetransistors, transient voltages as high as 500 volts may be utilized,thus providing current changes in a period as short as 2 microseconds.

Although the programming signal generator 44 responding to the scanmotor 68 has been shown as providing the switching signals, it is to beunderstood that other means may be utilized for developing the switchingsignals of the waveform 90. For example, digital techniques may beutilized with a rotating memory drum containing binary numbers storedthereon in a sequence indicative of the desired sequence of voltagelevels of the waveform 90. Read heads and amplifiers may develop digitalsignals as the memory drum rotates that are then passed through adigital to analogue converter to convert the binary nun hers to volt-agelevels such as indicated by the waveform of FIG. 3. Binary storagearrangements of this type are well known in the computer art and willnot be explained in further detail.

Thus, there has been described a current driving circuit that includes aregulating means having a negative feedback arrangement for providing apreselected steady state current to a load. During the presence ofswitching transients when changing the current through the inductance, atransient amplifier arrangement is provided to develop transient signalsfor efficiently increasing the resistance in the circuit during a veryshort period. Thus, current is rapidly changed through the inductance.This circuit is very useful, for example, for driving one or a pluralityof ferrite phase shifters such as may be utilized in radar scanning.

What is claimed is:

1. A circuit for driving current through an inductive load comprising apower source, driver means coupled in series with said inductive load,regulating means coupled across said power source and to said drivermeans for controlling the current therethrough, a source of switchingsignals of changing voltage levels coupled to said regulating means formaintaining a selected steady state current through said inductive'load,and transient amplifier pled in series with said load and said powersupply, said first variable impedance means having a control terminal,second variable impedance means coupled across said power supply andhaving a control terminal, a signal form-- ing resistor coupled inseries with said second variable impedance means and to the controlterminal of said first variable impedance means for developing signalsto control the current through said load, programming means fordeveloping switching signals having desired voltage levels indicative ofdesired current levels through said I load and coupled to the controlterminal of said second variable impedance means, and transientamplifier means coupled between said programming means and said signalforming resistor to develop transient signals to rapidly change thecurrent levels through said load.

3. A current driving circuit for an inductance comprising a power sourcecoupled across said inductance, a driver transistor having a controlelectrode and a load pathcoupled in series with said inductance, aregulating transistor having a control electrode and a load pathacoupled across said power source, a signal forming resistor coupled inSeries with the load current path of said regulating transistor, saidload current path coupled to the control electrode of said drivertransistor, signal generator means for developing switching signals ofpredetermined different voltage levels, said generator means coupled tothe control electrode of said regulating transi'stor for maintaining acurrent through said inductance in response to the steady state portionsof the switching signals, and transient amplifier means coupled to saidgenerator means and to the control electrode of said driver transistorfor rapidly changing current through said inductance in response to thetransient portions of said switching signals.

4. A current driving circuit comprising a first source of potentialhaving a positive and a negative terminal, an inductive load, a firstand a second resistor coupled in seriesv between the positive terminalof said first source of potential and said inductive load, a drivertransistor having a control electrode and having a load path coupledbetween said inductive load and said negative terminal of said firstsource of potential, a diode coupled in the load current path of saiddriver transistorbetween said driver transistor and the negativeterminal of said first source of potential, a regulating transistorhaving a control terminal and having a load path coupled between thepositive and, negative terminals of said first source of potential,asignal forming resistor coupled in the load path of said regulatingtransistor between said regulating transistor and the negative terminalof said first source of potential, said signal forming resistor alsocoupled to the control terminal of said driver transistor, a secondsource of potential having a positive and negative terminal, a tappedresistor coupled between the positive and negative terminal of saidsecond source of potential and with the positive'terminal of said secondsource of potential coupled to a point between said first and secondresistors, a stepping switch including a plurality of contacts coupledto said tapped resistor and a movable arm for contacting said contactscoupled to the control electrode of said reguilating transistor, meansfor moving said contact arm of. said stepping switch, an amplifiercoupled to said movable arm of'said stepping switch, and a high passfilter coupled between said amplifier and said signal forming resistor..

5. A circuit for supplying steady state current through an inductiveload at selected current levels and for rapidly changing the current.levels from one to another through the load comprising. a source ofpotential coupled across said load, variable impedance means coupled inseries with said load and having a control terminal, a diode coupledbetween said variable impedance means and said source of potential,regulating means and signal forming means coupled in series across saidsource of potential, said regulating means having a control terminal,said signal forming means coupled to the control terminal of saidvariable impedance means for maintaining said selected current levelsthrough said load, programming means for developing switching signalshaving a sequence of voltages at selected levels indicative of saidselected current levels and coupled to the control terminal of saidsignal forming means, and transient amplifier means coupled between saidprogramming means and said signal forming means for developing andapplying transient signals to said load at the time when the selectedvoltage levels of said switching signals change so as to change thecurrent levels through saidload in a short period of time.

6. A circuit for supplying current through an inductive load at aplurality of selected current levels and for rapidly changing from onecurrent level to another comprising a power source having first andsecond terminals, variable impedance means having a control terminal andhaving a load path coupled between said load andthe first terminal ofsaid power source, unilateral means coupled in said load path betweensaidvariable im--' pedance means and the first terminal of saidpowersource, current regulating means having a control terminal andhaving a load path coupled between the first and second terminals ofsaid power source, signal forming means coupled in the load path of saidregulating means and to the control terminal ofsaid variable impedancemeans, programming means for developing switching signals having aplurality of predetermined levels of voltages and coupled to saidcontrol terminal of said regulating means, said regulating meanscontrolling the current through said load in response to steady stateportion of said' switching signals, and transient amplifier meanscoupled between said programming means and said signal form-ing meansfor applying transient signals to said lead for rapidly changing thecurrent levels therethrough in response to the changes in voltage levelsof said switching signals.

7. A circuit for driving current through an inductance comprising apower source having a positive and nega-' tive terminal, a firsttransistor having base, an emitter and collector with the emittercoupled to a first end of said inductance, a first diode having an anodecoupled to the collector of said first transistor and a cathode coupledto the negative terminal of said power source, a

first resistor having a first" end coupled to a second endof saidinductance, a second resistor having a first end coupled to a second'endof said first resistor and having a second end coupled to the positiveterminal of saidpower source, a second transistor having a base, anemitter and a collector, a zener diode coupled between the emitter ofsaid second transistor and said positive terminal of said power source,a second diode having an anode and a cathode with the anode coupled tothe collector of said second transistor, a third resistor having a;first end coupled to the cathode of said second diode and to the base ofsaid first transistor, and having a second end coupled to the negativeterminal of said power source, a

second end of said first resistor, a plurality of tapping terminalscoupled to said. fourth resistor, a movable arm for contacting; onlyone: of said tapping terminals at atime and coupled to the base of saidsecond transistor, means coupled to said movable arm for controllingsaid arm to contact a desired sequence of said tapping terminals, anamplifier coupled to said movable arm, and a high pass filter coupled tosaid amplifier and to the first end of said third resistor.

8. A current driving circuit for an inductance comprising a power sourcehaving a positive and negative terminal, a first transistor having base,an emitter and collector with the emitter coupled to a first end of saidinductance, a first diode having an anode coupled to the collector ofsaid first transistor and a cathode coupled to the negative terminal ofsaid power source, a first resistor having a first end coupled to asecond end of said inductance, a second resistor having a first endcoupled to a second end of said first resistor and having a second endcoupled to the positive terminal of said power source, a secondtransistor having a base, an emitter and a collector, a zener diodecoupled between the emitter of said second transistor and said positiveterminal of said power source, a second diode having an anode and acathode with the anode coupled-t the collector of said secondtransistor, a third resistor having a first end coupled to the cathodeof said second diode and to the base of said first transistor, andhaving a second end coupled to the negative terminal of said powersource, a source of biasing potential having a positiveland a negativeterminal, programming means coupled to the second end of said firstresistor and to the base of said second transistor for developingswitching signals having selected voltage levels indicative of a desiredcurrent level through said load, said switching signals including directcurrent and transient portions and a transient amplifier coupled betweensaid programming means and said third resistor, whereby said secondtransistor is controlled to develop signals to control said firsttransistor during the direct current portions of said switching signalsand said transient amplifier is controlled to develop signals to controlsaid first transistor and to apply potentials to said inductance so thatsaid current levels through said inductance change from one to anotherin a short period of time.

9. A circuit for driving current through a load comprising a powersource, a first transistor having a load path and a control terminalwith said load path coupled be tween a first end of said load and thenegative terminal of said power source, a first diode coupled in seriesin the load current path of said first transistor between said firsttransistor and the negative terminal of said power source, currentsensitive means coupled between a second end of said load and thepositive terminal of said power source, a second transistor having acontrol terminal and a first and a second load terminal, a potentialsource coupled to the first load terminal of said second transistor, asecond diode coupled to the second load terminal of said secondtransistor, impedance means coupled between said second diode and thenegative terminal of said power source to form a current path, said baseof said first transistor coupled in said current path between saidsecond diode and said impedance means, programming means coupled to saidcurrent sensitive means for responding to current changes therethrough,said programming means developing a switching signal having a pluralityof desired voltage levels each indicative of a desired current levelthrough said load, said switching signals having steady state andtransient portions, said programming means coupled to the controlterminal of said second transistor for controlling current therethroughfor applying signals to said control terminal of said first transistorto maintain said desired current levels through said load as determinedby the steady state portions of said switching signal, an amplifiercoupled to said programming means, and a high pass filter coupled tosaid amplifier and to said current path between said impedance means andsaid second diode for passing current therethrough in response to thetransient portions of said switching signal to develop transient signalsfor changing the current level through said load in a short period oftime.

Vinding Apr. 5, 1960 Lippincott June 14, 1960

